Issue #3273143: Allow values of zero in XFDF

Fixes issue introduced in 8da26cad.

xfdf.inc

@@ -33,7 +33,7 @@ function create_xfdf($file, array $info, $enc = 'UTF-8') {
     $field_ele->setAttribute('name', $name);
 
     $value_ele = $field_ele->appendChild($doc->createElement('value'));
-    $value_ele->appendChild($doc->createTextNode($value ?: ''));
+    $value_ele->appendChild($doc->createTextNode((string) $value));
   }
 
   $ids_ele = $xfdf_ele->appendChild($doc->createElement('ids'));